Argon is recoverable from sources such as air and NH.sub.3 purge gas. Most argon is produced as a crude product from cryogenic air separation units because it is comparatively economical. However, the typical concentration of oxygen in crude argon produced by cryogenic air separation unit is 2-5%, whereas most of the argon uses require nearly oxygen-free argon. This leads to expensive downstream processing of crude argon to reduce its oxygen content. It is desirable to directly produce an argon stream from a cryogenic air separation unit in high recoveries with decreased oxygen content so that either it could be directly used by the users or minimize the processing required to further purify it.
Historically, most of the cryogenic air separation unit use a double distillation column of Linde-type with an argon sidearm column to recover argon from air, as disclosed in Latimer, R. E., "Distillation of Ar", Chemical Engineering Progress, 63 (2), 35-59 [1967]. FIG. 1 shows such a scheme. A carbon dioxide and water free compressed air stream is cooled and fed to a high pressure distillation column. This distillation column produces two liquid streams. The liquid nitrogen stream provides reflux for the top of the low pressure distillation column. The crude liquid oxygen stream from the bottom of the column is split into two fractions. One fraction is fed to the low pressure column as intermediate reflux. The other fraction is vaporized in the overhead reboiler/condenser of argon sidearm column and is fed to the low pressure column a few trays below where the crude liquid oxygen is fed in. The low pressure column produces gaseous nitrogen product, oxygen product and a waste nitrogen stream. An argon-rich (7-12% argon) vapor stream is withdrawn from the low pressure column, many trays below the vaporized crude oxygen feed point and is fed into the bottom of crude argon distillation column with a reboiler/condenser at the top. The nitrogen concentration of this argon-rich stream is typically very low (0.01 to 0.1% nitrogen). The vaporization of a portion of the crude oxygen liquid in the top reboiler/condenser nearly totally condenses the vapor rising to the top of the argon sidearm column, causing the condensate to flow down through the column, thereby providing the needed reflux. The argon available from the air is drawn as crude argon containing 2-5% oxygen from the top of the argon sidearm column.
Since argon is a valuable product, its recovery is often maximized by optimizing the number of theoretical stages in each section of the low pressure and argon sidearm columns and also the flowrates of various streams. The optimization of these theoretical stages goes hand-in-hand with the fact that since early 1930's sieve trays have been the trays of choice for cryogenic air separation unit. These sieve trays have certain contact efficiency and pressure drop per tray. The ratio of these parameters is the pressure drop (.DELTA.P) per theoretical stage (or equilibrium stage). The total pressure drop available for operation of the argon sidearm column limits the number of theoretical stages which can be used in it. The relative volatility of the argon with respect to oxygen (.alpha.) is about 1.5 at the bottom of argon sidearm column but is only about 1.1 at the top of this column. This low value of .alpha. at the top of the column makes it difficult to produce crude argon with low concentrations of oxygen in high recoveries.
As stated by Ruhemann, "we must consider that a high yield of argon is profitable as well as high argon concentration in the final product. Unfortunately these two conditions are irreconcilable." (see Ruhemann, M. "Separation of Gases", Second Edition, pp 223, Oxford University Press, 1949). This irreconcilable notion has plagued the cryogenic air separation industry (which uses sieve trays in its distillation columns) for quite a while; as a result, it has generally chosen a higher recovery (yield) of argon with significantly higher than desired concentrations of oxygen.
This oxygen-containing argon (crude argon) is then further purified in a catalytic reaction unit. In the first step of this purification scheme, crude argon is mixed with hydrogen and passed through a catalytic unit to react the oxygen to form water.
Recently, a process to produce a crude argon stream with lower concentrations of oxygen using sieve trays was disclosed in Soviet patent application (Belyakov V. P., et al., SU 1416820-A, 1988). In this patent application, the limitation of the total number of theoretical stages due to the total pressure drop available in the argon sidearm column is overcome by breaking this column in two zones. The first zone of this column contains enough sieve trays so that the pressure at the top is reduced to atmospheric. The gas stream from this zone upper part is warmed in a heat exchanger, compressed, cooled and fed at the bottom of the second zone of the side arm column. The oxygen enriched liquid stream from the lower part of the second zone is returned under pressure to the upper part of the first zone. An argon stream containing lower concentrations of oxygen is withdrawn from the top of the second zone. The problem with this arrangement is that it needs more capital for extra heat exchangers and a compressor. Furthermore, the use of a compressor increases the power consumption of the process.